The present invention relates to ultrasonic testing and more particularly to conformable acoustic couplers for use in ultrasonic testing.
A requirement exists to verify the existing chemical weapons stockpile declarations. However, a large part of the existing stockpiles are contained within sealed munitions. Typically, chemical munitions can be differentiated from conventional munitions by external labeling. However, many of these munitions, particularly in third world countries, are not labeled. In addition, munitions might be improperly labeled to conceal their contents. Identification of the contents by conventional analytical methods would be dangerous and time consuming. Heretofore, no methods are currently available to quickly and safely identify the contents of sealed munitions. However, a number of existing technologies appear to be adaptable to the task with some additional modifications. Ultrasonic technology is attractive because it is inherently safe, straightforward in application, has the potential to be miniaturized and is available at low cost. It is known in the art to conduct non-destructive testing of unknown materials within containers and the like by means of ultrasonic test procedures. One of several possible approaches to ultrasonic interrogation is to measure the velocity of sound in the various chemical agents. In this regard, U.S. Pat. Nos. 2,527,986 and 2,398,701 pertain to such ultrasonic testing techniques. It is generally well known that sound travels through different materials at different speeds. In the case of chemical verification, a specific fill might be related to a specific velocity at a measured temperature. While different chemical agents can be correlated to specific transmission velocities under controlled conditions, it has been found that differentiation of munition fills on the basis of velocity measurements is not practical. One reason is that transmission velocities are highly dependent on temperature, which is easy to measure but difficult to control in the field. In addition, the physical condition and purity of the chemical fills is known to vary significantly, and the variations encountered in the measurement of transmission velocities does not allow for reasonable statistical correlations. Also, the speed of transmission is similar in a number of unrelated materials and is not sufficiently unique for the intended purpose of verification of chemical stockpiles. In addition, several other practical constraints beyond temperature and fill purity require attention. The physical condition of the fills and the internal structural configurations vary, sometimes significantly, in munitions of similar type. In the case of filled munitions, information on the internal structure or condition is often not known. As an example, the burster core within the center of the munition can be short, long or of some intermediate length. It may be empty, packed with explosives, or filled with small explosive canisters. It is apparent that this high degree of variability among like munitions must be taken into consideration. Another approach to ultrasonic interrogation is the standing wave or resonance approach. In this regard see U.S. Pat. Nos. 4,215,583 and 3,861,200. Variations of resonant ultrasound have been used to test for structural integrity or homogeneity of various materials. In this method a structure is stimulated to induce one or more resonant frequencies and the frequency is compared with a reference spectrum for similarity. In the case of the present invention, the munitions would serve as the test structure and behave as an integral system. For a discussion of this method, one may see U.S. Pat. No. 3,595,069 and Postany, G.J., Influence of the Pulser on the Ultrasonic Spectrum: The Results of an Experiment"; Materials Evaluation, Mar. 1965, pgs. 417-419. When stimulated with broadband excitation, those materials with a higher resonance transmission will tend to dominate the frequency spectrum. In the testing of munitions, frequency transmission is favored by the shell or container casing as opposed to the fill. Similar shells or containers would tend to resonate in a similar manner, and the damping effects of the fill would become a secondary resonance effect. Different fills would have different damping factors, and thus can be differentiated. This approach is highly desirable for structural analysis of the munitions, but less so for determination of the contents of the fill because the fill level is observed to have a large effect on the frequency signature. In addition, the method in which the munition is supported and the position of the transducer both have an observable and sometimes unpredictable effect on the frequency signature. Such changes in the frequency signature make correlations with stored spectra difficult.
A third method of ultrasonic interrogation, the pulse-echo method appears to be somewhat more forgiving in subtle variations found in the resonance methods. One may see U.S. Pat. No. 3,595,069 and Kline, R.A. Measurement of Attenuation and Dispersion Using an Ultrasonic Spectroscopy Technique". J. Acoust. Soc. Am. 76(2), Aug. 1985, pgs 498-504, both of which are incorporated herein by references.
By this means, an acoustic transducer generates an acoustic pulse through an active surface which is thereby imparted to an article to be tested. The pulsed article produces a reverberation which is received by a sensor, usually a piezoelectric device. The sensor then produces a signal which is measured by an analysis device such as a computer. While such ultrasonic testing is very frequently used in articles having substantially flat surfaces, it has been a problem in the art to ultrasonically test curved surfaces. This is because it has been difficult to achieve a good acoustic energy coupling between the active surface of the transducer and the curved article to be tested. It is known in the art that when performing ultrasonic testing, close coupling should be attained between the active surface of a transducer and the article to be tested. Acoustic coupling to non-flat surfaces such as ammunition shells, storage tanks, etc., require coupling that easily conforms to its surface. One attempt at solving this problem has been to use an aqueous interface or grease as a coupling medium between the active surface and the article to be tested. In another attempt, plastic materials such as putty or modeling clay have been used. These have proved to be unsatisfactory since such materials leave residues which may be corrosive or absorb chemical agents. A method of close coupling by applying a fluid coupling layer is suggested by Pedrix, M., et al, Acoustic Independence Measurement by Reflection of Ultrasonic Impulse on a Specimen through a Coupling Layer", Transactions on Sonics and Ultrasonics, Vol. SU-28, No. 6, Nov. 1981. Coupling fluids are a problem when testing munitions since they tend to leave a residue and react with or absorb into the shell container.
For ease of operation and speed in testing a large variety of sizes and types of articles, a material is needed which does not leave a residue on the article to be tested. It also must be capable of functioning over a wide temperature range while being non-reactive, non-absorbing and not require subsequent cleaning of the tested article.
It is therefore an object of the invention to provide a conformable acoustic coupler for ultrasonically testing an article which quickly and easily conforms to a variety of non-flat surfaces and permits quick and efficient testing over a wide range of ambient conditions. Another object of the invention is to provide such a conformable acoustic coupler which does not leave a residue on the article which must be removed and where the coupler is non-reactive with the article.
These and other objects will be in part described and in part apparent from a consideration of the detailed description of the preferred embodiment.